Pressure sensitive adhesives based on non-thermoplastic hydrocarbon elastomers such as natural rubber, butyl rubber, synthetic polyisoprene, ethylene-propylene, polybutadiene, polyisobutylene, or styrene-butadiene random copolymer rubber, are well known in the art. The dominant means of processing such adhesives comprises masticating the elastomer on a two roll mill or in a Banbury type internal mixer, dissolving the elastomer and other adhesive components in a hydrocarbon solvent, coating the solution onto a backing, and drying the coated product to remove the solvent. This technology is discussed in Handbook of Pressure Sensitive Adhesive Technology, D. Satas (ed.), p. 268. Van Nostrand, N.Y., (1989). The solvent process has the disadvantages of being labor intensive, having low production rates, and emitting large amounts of potentially hazardous solvents to the atmosphere thereby requiring expensive equipment for solvent recovery and/or incineration. Moreover, such solvent based processes have become increasingly undesirable for use in making adhesive tapes because of increasing environmental and safety regulations throughout the world.
A processing method, sometimes used when a relatively thick adhesive layer is desired, comprises masticating the elastomer as described above, blending the rubber and other adhesive components in an internal mixer such as a Banbury mixer, and calendering the solid adhesive onto a backing using a three or four roll calender stack. The calendering process does not use solvent but does require very expensive equipment. Additionally, this process is slow, and is only economical when adhesive coatings greater than about 2 mils (51 .mu.m) thick are desired. An application of the calendering process is discussed in U.S. Pat. No. 2,879,547 to Morris.
Environmental considerations, lower initial capital investments, potentially higher production rates, and lower processing costs have led to accelerated interest in the use of continuous hot melt compounding and extrusion coating of thermoplastic adhesive compositions. The elastomers employed in this technique are "thermoplastic" elastomers of the block copolymer type, including for example, styrenic-diene block copolymers. Such materials exhibit a sharp reduction in viscosity at temperatures above 100.degree. C. where the styrene domains soften. Upon cooling, the domains reform and the material regains its rubbery character and cohesive strength. Illustrative teachings of adhesive formulations and processes of this type are found, for example, in U.S. Pat. No. 3,932,328 to Korpman, U.S. Pat. No. 4,028,292 to Korpman, and U.S. Pat. No. 4,136,071 to Korpman. The technology is further discussed in Handbook of Pressure Sensitive Adhesive Technology, pp. 317-373, D. Satas (ed.), Van Nostrand, N.Y., (1989).
Hot melt pressure sensitive adhesives based on these thermoplastic elastomers have found wide acceptance in the packaging, label, and diaper closure markets, but limited acceptance for use in paper masking tapes. The adhesive properties of pressure sensitive adhesives made from these thermoplastic elastomers differ from those of adhesives based on non-thermoplastic hydrocarbon elastomers, and are undesirable for many tape applications.
Because of their unique adhesive properties, non-thermoplastic hydrocarbon elastomer based adhesive systems, especially those employing natural rubber, are likely to be retained for many applications for which the thermoplastic elastomer systems are not adequate. Consequently, there is a need to provide a method of making adhesives from these non-thermoplastic elastomers which is environmentally appropriate, economically viable, and energy conserving.
Hot melt extrusion of pressure sensitive adhesives employing non-thermoplastic hydrocarbon elastomers such as natural rubber has been shown. However, low molecular weight plasticizing aids such as processing oils, elastomer oligomers, waxes, or other materials defined and described as plasticizers in Dictionary of Rubber, K. F. Heinisch, pp. 359-361, John Wiley & Sons, N.Y., (1974), are used as major components in the adhesive formulations. These plasticizing aids ease processing but detract from the ability of the finished adhesive to sustain a load and are generally known in the art to degrade adhesive performance.
Canadian Patent No. 698,518 to P. Beiersdorf & Co., discloses a solventless extrusion coating process for coating a PSA composition based on non-thermoplastic elastomers including natural and synthetic rubber, high molecular weight polyisobutylene and polyvinyl ether. The elastomer is pre-masticated and blended in a separate, batchwise operation using conventional rubber processing equipment such as a two-roll mill or a Banbury mixer. The preformed, compounded mixture is then fed to a heated single screw extruder and the molten coating is extruded onto a moving web. Plasticizing aids comprising up to 54% of the formulation are used. It is believed that these plasticizing aids are used to accommodate the coating difficulties normally associated with the extrusion of high viscosity elastomers.
Japanese patent application Shou 50-37692 to Fukugawa et al discloses a similar process of pre-masticating mixtures of ingredients of pressure sensitive adhesives for 25 minutes, supplying the premasticated mixtures to a heated extruder, extruding the materials at 230.degree. C. onto a substrate, and curing the extruded materials by exposing them to electron beam radiation to enhance the cohesive strength of the adhesive and improve the bond to the backing. This work describes a narrow range of materials including the non-thermoplastic elastomers natural rubber and styrene-butadiene rubber (SBR). In the two examples utilizing natural rubber, the natural rubber was blended with a styrene-butadiene elastomer and a plasticizing aid. The plasticizing aid equalled about 87.5% of the total rubber charge, and no tackifier resins were used. The non-natural rubber example included 25.8% plasticizing aid.
German provisional patent publication P-19 54 214.4 to Pyton AG discloses an extrusion process for the preparation of pressure sensitive adhesives which does not necessitate a separate pre-mastication step. A twin screw extruder is used to continuously compound and coat a formulation comprised of five different types of materials. Natural rubber and/or partially vulcanized rubber, latex, polybutene with a molecular weight between 70,000 and 200,000, and polyisobutylene with a molecular weight between 100,000 and 250,000 comprise the "cohesive component". Four other classes of ingredients are required to accommodate this process. These other ingredients include low molecular weight (less than 15,000) polybutene and polyisobutylene or native bitumen, reactive and/or non-reactive resins, antioxidants, and various metal oxide fillers. No specific compositions are taught, but the levels of plasticizing aids such as bitumen, or the low molecular weight polyisobutylene or polybutene are reported to range from 10-20%.
U.S. Pat. No. 2,199,099 to Cunningham discloses that air and oxygen enriched gases can be used to facilitate the oxidative breakdown of natural rubber in an internal mixer to reduce the molecular weight of the rubber. A continuous hot melt extrusion process that employs the air-assisted oxidative break-down of natural rubber followed by the addition of tackifiers and phenolic resin vulcanizing agent to form a thermosettable adhesive is known. In this process the molecular weight of the natural rubber is reduced to such a degree that when the phenolic resin is added, the combination of the rubber and resin can be processed at a temperature below that at which vulcanization occurs.
The hot melt extrusion of non-thermoplastic hydrocarbon elastomers has not proven to be a commercially practical method of making pressure sensitive adhesives having the properties needed for PSA tapes, such as masking, packaging and medical tapes. Furthermore such process technology is not envisioned for sustaining the dominant position of natural rubber elastomer, the single largest use of non-thermoplastic hydrocarbon elastomers for these PSA tapes. According to the Handbook of Pressure Sensitive Adhesive Technology, solvent and/or water coating of PSA adhesives are the only practical techniques for making such tapes, especially when the PSA's are based upon high molecular weight hydrocarbon elastomers. As discussed above, these techniques are not entirely satisfactory. Thus, it would be desirable to provide a practical method of compounding non-thermoplastic hydrocarbon elastomers at molecular weights and compositions of interest to the PSA industry.